1. Field of the Invention
The invention relates to an optical recording and reproducing apparatus for optically recording signals on a recording medium and for reproducing the recorded signals using a light source such as a laser, and more particularly to a method and an apparatus for adjusting a target position for focus control so that the focusing condition of a light beam irradiated on the recording medium is made to conform to a prescribed focusing condition.
2. Description of the Prior Art
As a conventional apparatus, there is an optical recording and reproducing apparatus which focuses a light beam generated by a light source such as a semiconductor laser on a disk-like recording medium that is rotating at a prescribed rotational frequency, to record signals on the recording medium or reproduce the recorded signals. On the recording medium is provided a minute track of 0.6 .mu.m width and 1.5 .mu.m pitch, spirally or concentrically. In order to record signals on the track or to reproduce the signals recorded on the track, the light beam irradiated on the recording medium is focus controlled so that the light beam satisfies a prescribed focusing condition.
A focus control system of the apparatus consists of a light detector which detects out-of-focus signals corresponding to the focusing conditions of the light beam on the recording medium by the light reflected from the recording medium or the light transmitted through the recording medium, and an actuator which moves a focusing lens for focusing the light beam on the recording medium in a direction almost vertical to the recording medium, and a control circuit which drives the actuator according to the out-of-focus signals detected by the light detector so that the light beam on the recording medium is controlled to focus in the prescribed focusing condition. In order for the light beam on the recording medium to be focus controlled so as to conform to the prescribed focusing condition, it is necessary to adjust the target position of the focus control.
The target position of the focus control system (hereafter called the target position) is conventionally adjusted so that the amplitude of the reproduced signal for adjustment recorded in advance on the recording medium is maximized, as stated in the laid open Japanese patent No. 61-14575/1986.
The method of adjustment will be further described in detail. When the recording medium is caused to rotate and the focus control is operated, the adjustment signal previously recorded is reproduced, the reproduced signal is envelope detected and held at peak, and the light detector is actuated by a micrometer, etc., thereby adjusting the target position of the focus control to the prescribed optimum location to maximize the voltage held at its peak.
In the conventional apparatus, in order to accomplish he focus control of a light beam at an optimum target position, the amplitude of the reproduced signal was measured and the focus control system wa adjusted to maximize the amplitude. However, the point where the amplitude of the reproduced signal becomes a maximum varies due to the effect of noise, etc., and the vicinity of the maximum amplitude point has less variation, in the amount of reproduced signal. Therefore, it was not practically easy to find the maximum amplitude of the reproduced signal due to the limitations in the accuracy of the measurement, and it took time to make adjustments.
In addition, if any vibrations or impacts are applied externally or if the components, etc., of the optical system are deformed after, extended use, causing the light detector to move by a minute distance while the apparatus was in use, the target position of the focus control becomes incorrect and the light beam cannot be focused precisely on the recording medium. If the recorded signals are to be reproduced under such conditions, deterioration of the quality of the signals resulted, degrading the reliability of the apparatus.